This invention relates to a drive ratio control system for a change speed gear transmission driven by an automotive vehicle engine, having a gear ratio selector through which the transmission is conditioned for forward drive and reverse travel, with automatic gear ratio shifting during forward drive in response to changes in output speed and load demand as reflected by the position of the engine fuel control or accelerator pedal.
According to the prior art, change speed gear transmissions for motor vehicles are automatically shifted through a range of available forward drive ratios in dependence on output speed and load demand determining a plurality of shift points, load demand being reflected by the positioning of the accelerator pedal between full load (kick-down) and idling positions. Certain of such forward drive ratios may be excluded from the gear shift sequence under control of the driver by means of a selector, during downhill travel for example. Besides the standard forward drive, neutral and reverse travel positions of such a selector, additional selector positions have been provided to correspondingly limit automatic shift to low, second and third speed ratios.
The foregoing types of automatic transmission control systems are capable of providing flexibility for the driver in connection with gearing having relatively few available gear ratios. In connection with multi-speed gear transmissions having a larger number of available gear ratios, both high and low gear ratios of the automatic shift range may be excluded by displacement of two selector levers, to achieve desirable travel conditions under idling and partial load demand positions of the fuel control pedal. According to yet other known automatic transmission control system, changes in shift points for fuel economy purposes is achieved during vehicle travel by means of a selector.
In addition to the selective exclusion of low and high speed ratios at opposite ends of an automatic shift sequence of available speed ratios, as aforementioned, it has also been proposed to select a sequence of speed raios under a predetermined shifting mode in accordance with actual vehicle acceleration to reduce the number of available speed ratios utilized. For example, in a 16-speed gear transmission, the drive ratio range utilized may consist of fewer speed ratios by skipping some of the 16 available speed ratios, with those speed ratios utilized being spaced from each other either equally or unequally, or some combination thereof predetermined by the shift pattern selected from a data storage file. Thus, depending on the drive conditions in the drive train of the entire transmission, fewer speed ratios may be utilized to reduce the amount of gear shifting operations.
None of the aforementioned automatic control systems are satisfactory for gear transmission arrangements in which the drive train and engine relationships are designed to favor economical fuel or energy consumption. For such fuel efficient arrangements, the maximum engine torque often accommodates the desired acceleration to be achieved under timely and sensible gear shift operation. With respect to the usual automatic gear transmission having relatively few speed ratios, the duration between gear shift operations varies with vehicle speed for which reason the frequency of shift presents no problem. For transmissions having a large number of available speed ratios, proper separation between the high speed ratios at one end of the gear shift sequence does reduce the frequency of shifting operations, but imposes relatively high demands on the driver and limits automatic adaptation of the control system to different driving conditions. Unsatisfactory adaptation to driving conditions associated with such automatic gear shift operations arises because of the inflexibility of the sequence of speed ratios over the entire drive range of the transmission while the frequency of shift operations is being reduced. The sacrifice in adaptability to driving conditions does not justify the reduced frequency in automatic shifting operations.
It is therefore an important object of the present invention to provide a transmission control system which achieves optimum adaptation of the drive train to the engine characteristics for favorable fuel or energy consumption while maintaining effective skip of available gear speed ratios to reduce the frequency of automatic shifting operations under different travel conditions, and without any sacrifice in the wear and tear of parts to thereby increase the life span of the gear shifting elements.